Condensation running down my range vent into my microwave in cold weather? (And cracks in ceiling)

[jeff47]

16. It looks like the cracks are still a heat expansion issue

The lumber also expands and contracts with heat. So you've got drywall, then you should have a vapor barrier, then insulation and lumber.

The lumber's cold, so it shrinks. That moves the drywall panels closer together. But the drywall is hot, so it expands. Since it can't expand within the plane of the wall, the pressure forces it to move in the other plane and you get a crack.

Now, that's assuming that the drywall covers the entire ceiling. If you have exposed beams, they may have only drywalled between the beams, and you are talking about cracks where the drywall meets the beam. If that's the case, you probably have a differential expansion issue - drywall expands and contracts at a different rate than lumber. To fix those, clean out anything loose in the crack, and then fill it with painter's caulk, also known as acrylic caulk. The caulk is flexible enough to deal with the differential expansion, whereas drywall compound is not. Use a putty knife to make the surface of the caulk flat so it looks like the drywall runs to the beam. Do not use silicone caulk, like you would use to seal a shower. Silicone caulk does not paint well and you can not use a putty knife to flatten it - it's too sticky.

As for the bow in the lumber, that's probably fine - every single piece of lumber used to build your house is bowed. Construction-grade lumber is never straight. The bow starts to become a concern when it's bowed a large distance, but that would be producing regular canyons instead of mini canyons - the gaps would be pretty huge and misaligned significantly.

On vapor barrier:
It would be difficult to see the vapor barrier through a crack - it's not really wide enough to get a good view. The best way to check is to go to the attic, and pull out some insulation and look*. You'll either see the back of the drywall, or a vapor barrier. If you don't know the difference, the vapor barrier should be loose enough to feel like it moves a tiny bit if you push on it, whereas the drywall will be completely solid. If you are still unsure, pull out more insulation until you have passed where two pieces of drywall meet - the drywall is either 4 foot by 8 foot or 4 foot by 12 feet. If you can't see that there are two pieces of drywall at a joint, you have a vapor barrier.

*Note that if your insulation is faced fiberglass batts you will not be able to pull it out - the batts overlap on the drywall-side of the lumber and are stapled there. If you have fiberglass insulation and it does not come up with a gentle tug, you can assume it's faced with a vapor barrier.

If you have a lot of dampness around the beam itself, they may have failed to properly install the vapor barrier across the beam. If the beam is covered with drywall, the only way to check is to remove the drywall, unfortunately. The vapor barrier should either cover the beam without a seam, or if there is a seam the vapor barrier should overlap by several inches, and be firmly attached so that it only wiggles slightly. Also if there's a seam, it should be taped.

If they are exposed beams, it's a lot harder to properly do the insulation and vapor barrier - you don't want vapor barrier as the finished surface of the beam, and the beam extends into the cold zone. To to it minimally, the vapor barrier should go a good distance up the beam in the attic and be firmly attached to the beam with staples, and then taped to the beam. To do it extremely well, keep the entire beam on the "warm" side by running the vapor barrier up and over the attic side of the beam, and then add rigid foam insulation to the attic sides of the beam.

I will also see about trying the shop vac on the vent. Hubby did the install, so we're wondering if something is slightly misaligned somewhere.

Probably not. It isn't like the pipes just butt together - One pipe fits inside the other for a significant distance. To have them separate requires a whole lot of movement.

Easiest fix would be to tape any joints with duct tape** and wrap the pipe with insulation. Hubby doing the installation is actually helpful - he knows how to take the microwave off so you can check the vent door and clean out that end of the pipe if necessary. I'd start with taping joints and pipe insulation in the attic where you can reach the vent pipe. That will likely fix it.

**Actual "Duct Tape" is not the waterproof tape that people normally call "Duct Tape". That's actually "Duck Tape" - as in the animal that quacks. However, "Duck Tape" is trademarked, so other companies used other names, and people settled on erroneously calling it "Duct Tape". Anyway, do not use it on ducting. It can not handle heat. The HVAC aisle of your local hardware store will have some actual Duct Tape. It vaguely resembles foil with an adhesive on one side. I prefer the kind where you have to remove a plastic backing to install the tape - the adhesive is thicker and thus does a better job of sealing. But it is likely that any of them will be up to the job.